1. Field of the Invention
The present invention relates, in general, to a polypropylene fiber and, more particularly, to a polypropylene fiber which is useful as a material for non-woven fabrics, thereby allowing the non-woven fabrics to be smooth and excellent in strength and providing workability and physical properties for the non-woven fabrics during after-processes. Also, the present invention is concerned with a method for preparing such fibers.
2. Description of the Prior Art
In order that staple fibers are prepared from polyolefin polymers, they have to undergo a series of processes: the polyolefin polymers are generally compounded with some amount of additives and the resulting mixtures are melt-extruded in ordinary commercial processes to give fibers, which are crimped and cut into predetermined lengths.
When being applied for the making of non-woven fabrics, polyolefin staples are typically processed in a carding machine to give non-woven webs which are then thermally bonded.
For thermal bonding, a pair of calender rollers, ultrasonification, or hot air is usually used.
Particularly as for polypropylene filaments or staples, they are arranged after opening and carding processes, and bridged to afford webs. These webs are thermally bonded with the aid of a calender roller with diamond or delta type patterns to produce non-woven fabrics which are industrially useful in various fields. Alternatively, hot air may be utilized instead of calender rollers. In this case, after being allowed to undergo a carding process, webs are bonded to give non-woven fabrics by means of hot air which is circulated in a porous drum.
Polypropylene non-woven fabrics find numerous applications in the disposable diaper, diaper for patients suffering from urinary incontinence, hygienic band, mask, and medical fabric industries. Although not demanding strength as high as that of woven fabrics, the non-woven fabrics used in these purposes have to be soft and satisfy the requirement of safety to skin because they are in direct contact with the skin.
The strength of non-woven fabrics varies depending on their preparation processes as well as on physical properties of material fibers.
With the aim of improving productivity, non-woven fabric producing manufacturers generally try to make high production speed. The high production speed, however, demands more excellent physical properties for the fibers for non-woven fabrics.
Leading to the present invention, the intensive and thorough research on polypropylene yarns or staples suitable for non-woven fabrics, repeated by the present inventors aiming to overcome the above problems encountered in prior arts, resulted in the finding that isotactic polypropylene homopolymers, which are found to have two endothermic peaks as measured by a differential scanning calorimeter (DSC), allow the production of novel fibers, which have been not yet reported in any arts, and guarantee the excellent strength and softness of the non-woven fabrics prepared from the fibers. In addition, the fibers of such a structure were found to be obtained by controlling melting indexes and polydispersity indexes in each process step through the total procedure.
Therefore, it is an object of the present invention to provide polypropylene fibers for non-woven fabrics, which can be applied to high-speed carding machines and guarantee the excellent strength and softness of the non-woven fabrics after thermal bonding.
It is another object of the present invention to provide a method for preparing such polypropylene fibers.
It is a further object of the present invention to provide a non-woven fabric prepared from such polypropylene fibers.
In accordance with an embodiment of the present invention, there is provided a polypropylene fiber, which is obtained from an isotactic polypropylene homopolymer with an isotactic index of 90 to 99% through melt-scanning or through drawing after melt-spinning, and shows two differential scanning colorimeter (DSC) endothermic peaks between 155 and 170xc2x0 C.
In accordance with another embodiment of the present invention, there is provided a method for preparing polypropylene fibers, comprising the steps of: (a) melting an isotactic polypropylene homopolymer with an isotactic index of 90-99%, a melting index (MIa) of 10.0-40.0 and a polydispersity index (PIa) of 2.5-6.0 to give a melt with a melt index (MIb) of 10.1-41.0 and a melt index (PIb) narrower by 10% or less than the PIa, the ratio of MIb/MIa ranging from 1.01 to 1.50; (b) spinning the molten polymer to produce fibers with a melting index (MIc) of 16.5-80.0 and a polydispersity index (PIc) narrower by 20% or less than the PIa, the ratio of MIc/MIa ranging from 1.65 to 7.50; and (c) optionally drawing the fibers.